1. Field of the Invention
This invention relates generally to a shuttle hook driver for a sewing machine provided with a stepping motor for driving a shuttle hook, and more particularly to such a shuttle hook driver in which rotation of each of a drive shaft of the stepping motor and a hook shaft of the shuttle hook relative to the other is damped.
2. Description of the Related Art
Sewing machines have conventionally been provided with a shuttle hook driver for driving a thread loop capturing shuttle capturing a thread loop in cooperation with a sewing needle. In a main shaft-linked type, the shuttle driver is driven in synchronization with a main shaft of the sewing machine driven by a sewing machine motor. On the other hand, shuttle hook drivers of the independent drive type have recently been put to an actual use. In the independent drive type, a dedicated stepping motor is provided for driving the shuttle independent of the main shaft, so that the shuttle is driven in synchronization with the main shaft and so that the rotation of the shuttle is controlled according to the varying sewing conditions. In the shuttle hook driver of the independent drive type, the stepping motor is generally disposed in a sewing bed. An end of a drive shaft of the stepping motor is connected directly to an end of the hook shaft by suitable coupling means. When the drive shaft of the stepping motor and the shuttle hook are thus connected fixedly together, the stepping motor is subjected to an inertia force of the shuttle hook via the hook shaft and the drive shaft at the time of speed change, for example, at the time of start and stop of the stepping motor. When the inertia force is increased, there is a possibility of loss of synchronism in the stepping motor.
To solve the above-described problem, the inventor of the present application proposed a shuttle hook driver of the independent drive type in which an elastic member is provided on a connecting member connecting the ends of the drive shaft and the hook shaft together. The elastic member is capable of transmitting rotating force or torque of the drive shaft to the hook shaft and performing a buffing action. The elastic member is elastically deformed such that the inertia force of the shuttle hook at the time of speed change in the stepping motor is buffed or absorbed, thereby preventing loss of synchronism of the stepping motor.
In the proposed shuttle hook driver, the inertia force of the shuttle hook can be lessened or buffed by the elastic deformation of the elastic member. However, an elastic energy is stored in the deformed elastic member. A rotating force of the shuttle hook due to the stored elastic energy sometimes acts in the same direction as the inertia force of the shuttle hook when the rotation of the stepping motor is stopped and when the stepping motor is rotated in the reverse direction immediately after the stop of rotation thereof. The stepping motor is subjected to a large resultant force, thereby tending to cause loss of synchronism.
Moreover, since the inertia force of the shuttle hook is further increased when the shuttle hook is driven at high speeds, the elastic member is elastically deformed to a large degree and accordingly, the elastic energy stored in the elastic member is increased. This further increases the rotating force of the shuttle hook due to the elastic energy. When the stepping motor is subjected to a resultant force of the increased rotating force, the stepping motor further tends to be desynchronized. Thus, the proposed shuttle hook driver poses a problem preventing a high-speed operation of the sewing machine.
The loss of synchronism can be considered to be prevented by increasing the size of the stepping motor. However, since the stepping motor is disposed in the sewing bed, to increase the size of the stepping motor is actually difficult.